Understanding DNA Vaccines
What are the major challenges that AIDS researchers have faced in developing DNA vaccines and how are recent advances helping them overcome these challenges?
Many common viral vaccines have been made by either killing a virus of interest or weakening it so that it doesn’t cause disease. When people are immunized with such preparations, they mount an immune response that subsequently protects them from pathogenic strains of the targeted virus. Unfortunately, using a weakened or attenuated version of HIV to stimulate protective immunity remains off limits to developers of AIDS vaccines. HIV mutates very rapidly, changing its genetic makeup dramatically even within one infected individual. Researchers therefore worry that an attenuated HIV could mutate and regain its ability to cause disease. Using a killed version of HIV in a vaccine candidate, meanwhile, is impractical because it is difficult to prove that the virus is completely inactivated. Further, such vaccines have failed to protect monkeys against simian immunodeficiency virus (SIV, the monkey equivalent of HIV).
These concerns have led scientists to look for better and safer methods for creating AIDS vaccine candidates. One such alternative is DNA vaccination, in which genes from a pathogen of interest are injected into people to generate a protective immune response. Essentially, DNA HIV vaccines are composed of harmless pieces of HIV’s own DNA that have been pasted into circular pieces of DNA known as plasmids, which infect bacteria in the wild and have long been used to express genes in laboratories.
After an engineered and purified DNA plasmid is injected into a person—usually with a gene gun into skin and muscle—it is passively taken up by cells. Those cells then use their own protein-making machinery to produce the HIV proteins encoded by the plasmid. This usually results in the activation of the cellular immune response, which targets virally infected cells. But DNA vaccines can also be engineered to elicit antibody responses, which can block the viral invasion of cells and have historically played a central role in vaccine immunization (see Feb. 2004 Primeron Understanding the Immune System, Part 1 and Mar. 2004 Primer on Understanding the Immune System, Part II).
When DNA vaccination was first proposed in the early 1990s, the preclinical data seemed promising. Scientists had found that mice inoculated subcutaneously with genes encoding human growth hormone developed antibodies against that protein. Further, DNA vaccine candidates were even then relatively easy to make and stable at room temperature. Researchers were therefore attracted to this strategy. It meant that such vaccine candidates could be produced relatively rapidly and cheaply in large quantities and would, further, suit the needs of the developing world, where refrigeration capacity is often limited and transportation difficult.
But DNA vaccine candidates also presented some challenges. Most prominently, they triggered relatively weak immune responses because plasmids are not very efficiently taken up by cells. Producing stable forms of engineered plasmid DNA also proved to be harder and more expensive than researchers had expected. These setbacks dampened enthusiasm for DNA vaccines, not just against HIV but other pathogens as well. In fact, no DNA vaccine has yet been licensed to prevent a human disease.
New tools improve responses
In recent years, however, technological advances have revitalized the field of DNA vaccination. One new tool that has contributed to its resurgence is electroporation (EP), a vaccine delivery technology that induces temporary pores in the membranes of muscle or skin cells so that they can more easily take plasmids. Small hand-held EP devices these days often include a needle to inject the vaccine and thin wires that administer short electrical pulses during vaccine delivery.
Initially developed in the 1970s, EP has been refined and tested in a growing number of human studies since the early 1990s. In recent years, EP devices have been tweaked to cause less pain and deliver plasmids more efficiently, and continue to be tested in HIV vaccine trials.
Adjuvants, which stimulate the immune response to vaccines, are also being used to improve DNA-based vaccine candidates. Many licensed vaccines, such as the influenza vaccine, are formulated with chemical adjuvants. But as researchers’ understanding of the immune system and its factors has grown in sophistication, entirely novel adjuvants and methods for their co-delivery are being tried out in clinical trials. Rather than just co-formulate their vaccine candidates with adjuvants, for example, AIDS vaccine developers have designed DNA plasmids to carry genes for proteins that are potent boosters of cellular immune responses. One such protein, Interleukin 12, is naturally produced by dendritic cells—which have long been known to play a central role in vaccine immunization. Clinical trials are now testing DNA vaccine candidates that are delivered via electroporation along with the gene for IL-12.
Researchers have also tweaked the plasmids used to make DNA vaccines so that human cells can express more of the HIV antigens they encode, and so trigger more robust immune responses. One way they do this is by including in the plasmids promotors—DNA sequences that initiate the reading of genes for protein production—that are more effective at driving gene expression.
Vaccine developers also enhance immune responses by using DNA candidates as a prime, and then boosting the response it provokes with another agent—such as the canarypox viral-vector vaccine candidate that was used in the RV144 trial in Thailand. Any such regimen is referred to as a heterologous prime-boost. The DNA used as the prime focuses the immune response on the vaccine candidate inserts, perhaps with the help of an adjuvant. The subsequent boost enhances the primed response.
Together, new technologies and such traditional immunization strategies have contributed to a resurgence in DNA vaccine development.